gordian knot an intricate knot tied by Gordius, the king of Phrygia, and cut by the sword of Alexander the Great after he heard that whoever undid it would become ruler of Asia
koan a paradoxical anecdote or a riddle that has no solution; used in Zen Buddhism to show the inadequacy of logical reasoning
pons asinorum a problem that severely tests the ability of an inexperienced person
problem -
a state of difficulty that needs to be resolved; "she and her husband are having problems"; "it is always a job to contact him"; "urban problems such as traffic congestion and smog"
A problem is an obstacle which hinders the achievement of a particular goal, objective or purpose. It refers to a situation, condition, or issue that is yet unresolved. In a broad sense, a problem exists when an individual becomes aware of a significant difference between what actually is and what is desired between one or more individual.
OmegaWiki Dictionary
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problem A difficulty that needs to be resolved or dealt with.
problem A question with the explicit purpose of being solved by the students.